1. Field of the Invention
The present invention generally relates to a capacitor, a semiconductor device including the capacitor, and a method of manufacturing the semiconductor device, and more particularly to a nonvolatile semiconductor storage device including a capacitor using a ferroelectric material as a dielectric.
2. Description of the Related Art
Some semiconductor devices, particularly, semiconductor memories, have a variety of properties. The semiconductor memories are roughly classified into volatile memories that lose information stored therein when power is turned off and nonvolatile memories that retain information stored therein when power is turned off. A nonvolatile memory that retains data in a capacitor using a ferroelectric material as a dielectric is referred to as a ferroelectric random access memory (FRAM®).
The FRAM uses the two remanent polarization properties of different polarities of a ferroelectric material so as to retain data when power is turned off and no electric field is applied across the capacitor. The FRAM can rewrite data 1010˜1012 times, which is far more than the number of times (106) a flash memory can rewrite data. The FRAM can rewrite data at high speed in tens of nanoseconds.
In the FRAM, the ferroelectric material, which is material for the dielectric of the capacitor, is polarized in one of two directions upon application of an electric field, and remains polarized in the same direction even after removing the electric field. Data of logical “1” and “0” corresponding to one and the other, respectively, of the polarization directions can be recorded in the FRAM by distinguishing between the polarization directions. Further, the polarization direction can be switched from one to the other by applying a sufficient electric field in the direction opposite to the polarization direction.
Typical ferroelectrics include lead-based ferroelectrics such as PbZr1−xTixO3 (0.0≦x≦1) (PZT) and Pb1−yLayZr1−xTixO3 (0≦x, y≦1) (PLZT) and bismuth-based ferroelectrics such as SrBiTa2O9 (SBT).
Generally, in PZT, remanent polarization is reduced as the reversal of polarization is repeated, causing property degradation (fatigue phenomenon). Meanwhile, the FRAM, which uses one of such ferroelectrics, is employed in apparatuses handling personal or money information, such as IC cards. Accordingly, the FRAM is required to be highly reliable. In order to realize the expected durable period of ten years, it is desirable that the FRAM have a larger remanent polarization charge.
However, most conventional ferroelectric capacitors manufactured by spattering or the sol-gel method have a remanent polarization charge of 20-25 μC/cm2. Only tens of percents of the conventional ferroelectric capacitors satisfy a remanent polarization charge of 30 μC/cm2 required for a product. FIG. 1 is a diagram showing a conventional ferroelectric capacitor 100 of the FRAM. Referring to FIG. 1, the ferroelectric capacitor 100 is formed by successively layering a Pt film 101 as a lower electrode, a PZT film 102 that is a ferroelectric film, and a Pt film 103 as an upper electrode in the order described on, for instance, a contact plug connected to a diffusion region of a transistor. The lower-electrode Pt film 101 is oriented in the [111] direction toward the direction of film thickness by its self-orientation characteristic. The PZT film 102 formed on the Pt film 101 is oriented in the [111] direction, affected by the orientation of the Pt film 101. Since the PZT film 102 has a polarization axis in the [001] direction, the direction of the electric field applied between the lower-electrode Pt film 101 and the upper-electrode Pt film 103 differs from the direction of the polarization axis of the PZT film 102. This results in the problem that an electric charge usable as remanent polarization decreases naturally.